1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic system, a photosensitive drum and an intermediate transfer belt (hereinafter, will be referred to as “ITB”) on which a toner image is borne are preferably driven such that a surface speed is set to be at a constant speed.
A reason therefor is that, when the surface speed of the photosensitive drum is fluctuated, a position to be exposed with light is shifted from a position on the photosensitive drum which is originally supposed to be exposed with light.
Another reason therefor is that also in a process for a primary transfer of a toner image formed on the photosensitive drum onto the ITB, in a case where an alternating current-like speed difference exists in the surface speed of the photosensitive drum and the ITB, a position of the toner image transferred onto the ITB is shifted from a position that is originally supposed to be transferred.
As a result, an image defect such as a color misregistration corresponding to positional misregistrations among respective colors or a banding corresponding to a periodic positional misregistration occurs on an image drawn on a recording paper.
For that reason, the speed of the photosensitive drum and the ITB is highly accurately controlled by controlling a speed of a motor functioning as a drive source for the photosensitive drum and the ITB by using a speed detection sensor.
A brushless direct-current motor (hereinafter will be referred to as “BLDC motor”) is used as the motor in many cases since the BLDC motor is inexpensive, silent, and highly effective. In addition, a hybrid stepping motor is also adopted since a speed fluctuation hardly occurs by a disturbance.
A method of controlling a rotation speed of the photosensitive drum or the ITB to be constant by using the stepping motor includes a feed forward control (hereinafter, will be referred to as “FF control”). A periodic rotation fluctuation component generated by an eccentricity of shafts of a motor gear and a reduction gear is measured. Subsequently, a profile is created on the basis of the measurement result.
In the FF control, a speed fluctuation component extracted from the profile is extracted, and a speed correction value for cancelling the speed fluctuation component is generated, so that the generated speed correction value is added to a target speed instruction value.
A plurality of speed fluctuation components exist in the driving of the photosensitive drum or the ITB. In the FF control in the related art, the plurality of speed fluctuation components are not highly accurately controlled. In view of the above, Japanese Patent Laid-Open No. 2008-278620 proposes a method of gradually decreasing the speed fluctuation by repeatedly performing the FF control.
According to the technology disclosed in Japanese Patent Laid-Open No. 2008-278620, the FF control is to be repeatedly performed until the speed fluctuation falls within a target speed fluctuation amount, and a problem occurs that this repetition takes long time.